Common desert organisms may help in search for life in space

ASU geomicrobiologist Ferran Garcia-Pichel doesn't just study
microorganisms, he studies how they interact with and change
their inanimate surroundings. In his work he searches for
clues about the first type of organism to dominate the early
earth continents -- clues he says lead him to believe their
descendants, still standing in the deserts of the Southwest,
could be similar to the last ones standing in the case of a
drastic change.

Well, maybe not standing. These tough little critters, which
thrive in desolate places and extreme conditions, have no
legs.

They are the microscopic, single celled cyanobacteria --
one of the oldest life forms on Earth. While they may no
longer dominate the planet, they are well known from their
marine fossil stromatolites, and they still live with us
here, forming soil crusts in the Arizona desert.

Along these lines, Garcia-Pichel and his collaborators will
be presenting several papers and posters at the 2003 NASA
Astrobiology Institute General Meeting at Arizona State
University. These are on the topics of cyanobacterial
community structure, microbial mats, desiccation under
different conditions, molecular analysis of calcifying
cyanobacteria, comparisons of rDNA sequences from
cyanobacteria, as well as the analysis of microbial
nitrogen cycling in desert soil crusts.

The Earth is about 4 billion years old. Fossilized
stromatolites (large deposits left by aquatic communities
of cyanobacteria), the most common fossils from the
Precambrian period, are known for about 3.5 billion years.
That's more than 75% of the earth's history. The size and
number of these fossils indicate that cyanobacteria once
formed the major ecosystems of the Earth. They were
witness to nearly every stage in the earth's evolution.

Geochemical evidence shows that cyanobacteria also seem
to have played a major role in transforming the early
earth into the earth we know today. They invented
oxygenic photosynthesis and through this turned the
earth's biosphere from reducing to oxidizing.

Studying fossilized cyanobacteria allows us to study what
life was like and how it evolved on what was in many ways
a different planet. But there may be more to the story.
According to Garcia-Pichel, their role in the
transformation of early life on land has not been duly
recognized, perhaps because terrestrial cyanobacteria do
not fossilize as readily as their marine counterparts.

Scientists might have to look at indirect evidence -- a
unique sort of chemical or mineral signature (known as
a biosignature) that an organism leaves in its
environment. The study of living cyanobacteria and the
chemical biosignatures they leave in their environment
is essential for the task of looking for past-life
evidence on other planets and our own.

According to Garcia-Pichel, if we can learn how to
recognize the signatures left behind by living terrestrial
cyanobacteria on our planet today, we will soon be able
to look for them in our geologic record, as well as on
other planets. The key to finding their evidence is in
figuring out their signatures.

Garcia-Pichel hypothesizes that, because they lacked
competitors, the early earth's surface may at one time
have been completely covered with cyanobacteria. But,
as the diversity of life increased, things changed. The
advent of plants with leaf-litter blocked much needed
sunlight from hitting cyanobacterial communities in the
soil. In short, they were crowded out.

And this brings Garcia-Pichel and his colleagues to the
deserts of the Southwest, where aridity limits plant
development, soils are usually bare and cyanobacteria
still thrive in communities known as soil crusts.

In some of the more pristine portions of the Arizona desert,
cyanobacteria essentially lead an existence of withstanding
desiccation and the insults of the environment, living one
or two millimeters under the surface. From time to time,
every monsoon or so, they get wet and come to the surface
for a couple hours of activity. In these wet periods they
multiply through cellular division and repair all the
damage from the dry period.

During dry spells, the cyanobacteria (as well as other
microbes that live within their community) just lay dormant
and suffer what Garcia-Pichel calls "the slings and arrows
of the Southwest." To minimize the effects of these slings
and arrows, they secrete slime. This slime creates a crust
that holds the soil of the cyanobacterial ecosystem in
place. This slime essentially cements the desert crust
and stabilizes the soil, keeping it from blowing away in
the wind.

Though cyanobacteria are too small to see with the naked
eye, we definitely see their effects. Dust storms in
the urban Arizona, as well as the unacceptably high
particulate matter count, are enhanced by the loss of
cyanobacterial soil communities from agriculture and
construction.

Garcia-Pichel and his colleagues plan to study modern
desert soil crusts, a common and important remnant
of cyanobacterial ecosystem, and how they undergo
diagenesis -- the chemical transformations that occur
after burial.

Cyanobacteria may be old, but the science and
methodologies Garcia-Pichel and his colleagues incorporate
in geomicrobiological research are new. According to
Garcia-Pichel we are just beginning to understand how
microbes can and probably do drive biogeochemical cycles.
"This area of geomicrobiology is really, in the Western
world, a novelty," says Garcia-Pichel, "people still are
amazed that microbes can do things with minerals and
rocks."

Novelty or not, Garcia-Pichel and his colleagues believe
geomicrobiology will help scientists understand the
evolution of the earth, and the possible evolution of
other planets in this solar system and beyond.

Speaking of the applications of their research to the
question of Mars, Garcia-Pichel said, "If we postulate
that water was not always relegated and then became
relegated, and that microbial life was present, what
kind of ecosystem would have been the last to be on
the surface soils of Mars? It would have been something
like desert crusts today. So just there our chances of
finding some indirect evidence of life are highest, if
we just know how to track, how to teat and how to
recognize possible biosignatures from these ecosystems."

The NASA Astrobiology Institute is composed of over 700
researchers distributed at more that 130 research
institutions across the United States. Its central
offices are located at NASA Ames Research Center, in
the heart of Silicon Valley, California. Additional
information about the NAI can be found at its website:
http://nai.arc.nasa.gov